1. Field of Invention
The invention is a ramp for reducing the noise generated by the supersonic flow exhausting from a nozzle and, more particularly, includes a single expansion ramp having a porous surface for attachment to the exhaust opening of a fluid nozzle.
2. Description of the Prior Art
A series of shock waves are created by the exhaust gases of a jet engine when those gases reach supersonic velocity downstream of the choke plane of the exhaust nozzle. For an aircraft being propelled by several jet engines, shock waves are created by each jet engine even when the aircraft is travelling at subsonic speeds, for example, when the aircraft is taking off or landing. This poses a problem because take-offs and landings are frequently from airports located quite close to residential areas and thus disturb the residents and are a source of constant friction between the airlines and such residents. The nuisance created by shock wave noise from jet exhausts has caused alteration of take-off and landing patterns at many airports in order to meet federal regulations and, in some cases, even stricter local noise abatement ordinances.
It is also desirable to reduce shock wave noise from jet exhausts in order to lessen the deleterious effect such noise has on the structural integrity of the aft section of an aircraft.
There are two major components to shock wave noise: screech and a broadband component. Both components are identified in FIG. 1, which is a graph of sound pressure level versus frequency for the noise generated by the exhaust of a jet engine. Screech is characterized by the high amplitude spike at a frequency lower than the broadband component. The broadband component extends over a broader range of frequencies and at a lower sound pressure level than screech.
Screech is a resonant feedback instability in a supersonic plume containing shock cells. An acoustic wave travelling upstream outside the plume excites the shear layer at the nozzle lip. A shear layer instability wave then convects downstream and interacts with the standing shock waves in the plume, causing the plume to oscillate from side to side and creating the sound waves comprising the screech component. In subsonic flight, the screech sound waves travel upstream to the nozzle lip, where they excite another shear layer instability wave. The frequency at which the screech component occurs is the resonant frequency for the aforementioned feedback loop.
It is known in the prior art that the screech component can be eliminated by extending a ramp having a non-porous surface from the nozzle exhaust opening to stabilize the plume and prevent it from oscillating.
The broadband component is created by turbulent vortices passing through the standing shock waves in the plume. There is no feedback and thus there is no single resonant frequency. That is why the frequency band of this source of shock wave noise is much broader than the screech component. However, as this component does not rely on feedback from downstream fluid flow, it is not reduced by placing a non-porous ramp extension on the nozzle exhaust opening.
The prior art also reduces shock wave noise by using a number of small nozzles in place of each large nozzle. This results in shifting the broadband component to a higher frequency range, where it is easier to absorb with conventional sound absorption materials.
The drawback to this approach is that the added number of nozzles requires more material and associated hardware, and thus adds weight to the propulsion system. Additionally, using a number of smaller nozzles results in a total inner surface area facing the exhaust flow which is greater than the comparable inner surface area for a single nozzle, and thus results in greater skin friction drag, which lowers the velocity of the exhaust gases and concomitantly reduces the thrust of the jet engine. Furthermore, using a plurality of smaller nozzles creates low pressure areas between them, which lowers the base pressure and thus increases the base pressure drag of the aircraft. This solution thus sacrifices performance to reduce shock wave noise.
Shock wave noise has also been reduced by weakening the shock waves emanating from the exhaust nozzle by using a nozzle having a variable configuration. The shock waves are weakened when the design parameters of the nozzle approximate the exhaust flow parameters. The configuration of the variable configuration nozzle is changed during flight depending on the flight conditions so that the design point of the nozzle becomes closer to the current parameters of the exhaust flow. However, variable configuration nozzles have inherent drawbacks in that they weigh more than a nozzle having a fixed configuration, are mechanically complex, and must be continually inspected and maintained to ensure proper operation.